Of the four Galilean satellites Io is the closest to Jupiter. It is also the most volcanically active object in the solar system with an atmosphere rich in SO2. Along the orbit of Io lies a torus of plasma comprised of ionized material that escapes from Io's atmosphere and becomes trapped in the jovian magnetosphere. Küppers and Schneider used the Kitt Peak telescope to obtain spectra of the torus in the near infrared and detected a 2% abundance of Cl+ in the torus. Chlorine on Io may be derived from HCl or Cl2 gas or from molten SCl2 released during volcanic eruptions. Another possible source of chlorine may be NaCl, either in solid deposits brought to the surface via SO2 aquifers or in gaseous form produced by atmospheric reactions between Na- and Cl-containing compounds. New models that combine these measurements of Cl with previous studies of Io's chemistry may allow ground-based volcanologists to explain the dynamic processes as its interior is heated and extruded by its interactions with Jupiter.—LR